1. Field of the Invention
The present invention relates generally to image intensifier tubes used in night vision equipment and systems. The present invention relates more specifically to methods and systems for utilizing certain imperfect or blemished image intensifiers with night vision equipment using screen masks and ancillary screen elements.
2. Description of the Related Art
The core component in a night vision device is the image intensifier tube. These optoelectronic devices are manufactured to strict standards that frequently result in a high and expensive rejection rate of manufactured tubes that do not meet the standards for military (as an example) night vision equipment. It is not unusual to have blemishes in the nature of emission points or e-points in the resultant screen associated with the image intensifier tube that can jeopardize the “clean” quality of the image.
An image intensifier tube typically displays an image through the use of a fiber optic bundle. The output screen of the fiber optic bundle is ground and polished either flat or concave to accommodate or match the image gathering characteristics of the night vision system eyepiece that is used to view the image. Because of hard to control process variables in the manufacture of image intensifiers, they often have blemishes and emission points that make them un-useable, or at best difficult to sell. This is understandable because in general, a purchaser does not like to see imperfections in the field of view and generally considers such imperfections as a sign of a less than perfect tube. This is especially true given the high cost of night vision systems in general. Typically, blemished image intensifiers are a tough sell and usually a manufacturer must dramatically discount a night vision system that incorporates or utilizes one of these so-called “blem-tubes”.
Most night vision equipment has specifications that require the absence of blemishes of a certain size within one or more zones within the typically circular image field. Reference is made to FIG. 9 for an understanding of the manner of grading an image intensifier tube with one or more blemishes. For example, Zone One 112, the center of the circular screen field 110, would typically require the absence of any blemishes of significant size, mainly because it provides the focal point for the view that the user is directed towards. A concentric Zone Two 114 surrounding Zone One may have a requirement of no more than one or two blemishes larger than 2 μm in diameter. A further concentric Zone Three 116 forming the edge of the circular image field 110 may have less stringent requirements such as no more than two or three blemishes larger than 5 μm or less. The above numbers are given as examples only and do not constitute the specific requirements for any particular night vision device. These example requirements do, however, convey the importance associated with the quantity and quality of the blemishes and their position within the field of view formed by the image intensifier.
As a result of the above requirements, many manufactured image intensifier tubes are discarded as defective and inappropriate for use in standard night vision goggle equipment. Typically these defective units can not be repaired and are simply disposed of even though they may present a field of view that is relatively free of blemishes and might operate satisfactorily for many or most purposes. It would therefore be desirable to have devices and methods for utilizing these so-called “defective” image intensified tubes so as to reduce the waste associated with a quite costly manufacturing process.